Digital aerial photographs were collected for the PAMAP program in two cycles, covering the years from 2003 through 2006 and from 2007 through 2008, respectively. The Cycle 1 and Cycle 2 maps shown below indicate the counties that were flown each year. Cycle 1 covered all of Pennsylvania except for the five southeasternmost counties. Cycle 2 included counties in the eastern third and south-central part of the state. The aerial photography was leaf-off, natural color. The photographs were taken from an altitude of 10,000 feet above median terrain at a negative scale of 1:19,200.
Following PAMAP specifications, all of the photographs were orthorectified. Orthoimages combine the image characteristics of a photograph with the geometric qualities of a map. Aerotriangulation utilized GPS ground control points, airborne GPS data, and image coordinates to orient each digital photograph and correct for measurement errors and distortions caused by terrain relief and sensor position. The orientation parameters obtained from the aerotriangulation process were used to project each pixel of the image into the Pennsylvania State Plane North or South Coordinate System appropriate for the county flown, and the relevant PAMAP digital elevation models were used to correct for relief displacements.
All orthophotographs were checked for geometric accuracy, image quality, and tonal balance. The PAMAP orthoimages have a horizontal scale accuracy of 1:2400 and are not meant to be used at finer scales. All of the images have a pixel resolution of 1 foot, except for the 2003 photographs, which were produced with a 2-foot pixel resolution.
The orthophotographs were mosaicked together and then clipped to the PAMAP tiles. The resulting GeoTIFF files have names that start with a concatenation of the first four digits of the State Plane northing and easting that defines the northwest corner of their tile. This number is followed by the state identifier “PA” and the State Plane zone “N” or “S.” There is about one tile of overlap between the two zones.
Most GeoTIFF files are 286 MB in size, but the GeoTIFFs from the 2003 collection are approximately 72 MB each. Many of the photographs are also available as compressed MrSID or JEPG2000 images. The file sizes of the compressed images are at least an order of magnitude smaller than their corresponding GeoTIFF files.
After the initial release of the orthophotographs, several data issues were reported and addressed. A record of these issues is available through the PAMAP Documents web page, as are horizontal accuracy assessment reports for digital orthophotographs from various counties and smaller areas.
Obtaining the Data
The orthoimages are organized into datasets by collection year. You can select and download the PAMAP orthoimages free of charge through the PAMAP Data Download Portal. The images are stored on servers maintained by Pennsylvania Spatial Data Access (PASDA) and can also be downloaded from the PASDA website (http://www.pasda.psu.edu/). All data use the NAD83 horizontal datum, GRS80 ellipsoid, NAVD88 vertical datum, and GEOID03 National Geodetic Survey.
All PAMAP orthoimagery are also available for bulk and/or offline acquisition from the Pennsylvania State Data Center.